This application claims priority of Japanese Patent application number 2001-023300, flied Jan. 31, 2001.
1. Field of the Invention
This invention relates to an optical fiber transmission line in which chromatic dispersion is controlled.
2. Background of the Invention
In a long haul optical fiber transmission system, dispersion compensating fibers are disposed at appropriate intervals because it is necessary to control accumulated chromatic dispersion within a predetermined value (See U.S. Pat. No. 5,361,319).
In wavelength division multiplexing (WDM) optical transmission that has attracted public attention as a means to increase a transmission capacity, there is another problem that accumulated chromatic dispersion differs per wavelength since chromatic dispersion of a transmission optical fiber differs per wavelength (this is called as a dispersion slope). Although a configuration to compensate the difference of accumulated chromatic dispersion values between the wavelengths at a receiver or transmitter side is proposed, the dispersion amount that the transmitter or receiver side can compensate is limited. In addition, the permissible difference of dispersion values tends to decrease as a bit rate per channel increases.
Such an optical transmission line has been proposed that locally compensates the accumulated chromatic dispersion per optical repeating span and widely compensates the accumulated chromatic dispersion per predetermined number of optical repeating spans simultaneously (See, for example, Japanese Laid-Open Patent Publication No. 2000-82995, T. Naito et at., ECOC ""99 PDPD2-1, Nice, 1999, and EP 1035671 A2).
In the configurations disclosed in the Japanese Laid-Open Patent Publication No. 2000-82995 and paper by Naito et al, when an optical fiber having the dispersion value between xe2x88x9220 ps/nm/km and xe2x88x9245 ps/nm/km is used as a negative dispersion fiber, the ratio of the length of the negative dispersion fiber to a positive dispersion fiber increases. Consequently, optical input power given to the negative dispersion fiber having a relatively small effective core area increases and accordingly signal degradation due to the nonlinear effect also increases.
In the configuration disclosed in EP 1035671 A2, since the local dispersion Dlocal is set to a positive value (between +1 ps/nm/km and +4 ps/nm/km), a dispersion compensating fiber to be disposed at a wide area compensating span must be a negative dispersion fiber. In consideration of practical maintenance of a system, it is preferable that the interval of repeaters should be 20 km or more and also the length of each repeating span should be approximately equivalent. However, if a negative dispersion fiber with a dispersion value of xe2x88x9250 ps/nm/km or less (absolute value is 50 ps/nm/km or more) is used for the compensation of the wide area, the length of approximately 10 km is sufficient and this is very different as compared to the lengths of other repeating spans. Therefore, to equalize the lengths of all repeating spans, it is necessary to provide a third optical fiber with a different chromatic dispersion value as a dispersion fiber for the wide area compensation, which means the to use of three kinds of optical fibers. This makes the maintenance of the system very difficult. For instance, when broken parts are to be connected, it is required to provide three kinds of optical fibers and insert one of the fibers after selecting a suitable one for the optical fiber with the broken parts.
In addition, since the effective core area of a negative dispersion fiber is small, it is necessary to decrease the optical input power to reduce the degradation of transmission performance due to nonlinear effect in the negative dispersion fiber at the repeating span for wide area compensation. For example, it is necessary to dispose an attenuator immediately in front of the negative dispersion fiber.
It is therefore an object of the present invention to provide a low nonlinear optical fiber transmission line in which two kinds of optical fibers flatten chromatic dispersion.
An optical fiber transmission line according to the invention consists of a plurality of local dispersion compensating spans, a wide area dispersion compensating span disposed at predetermined intervals, and optical repeating amplifiers to connect each span, wherein the local dispersion compensating span consists of a first optical fiber of positive dispersion having an effective core area of 130 xcexcm2 or more and a second optical fiber having a negative dispersion value of xe2x88x9250 ps/nm/km or less to transmit an optical signal output from the first optical fiber. The wide area dispersion compensating span consists of a third optical fiber having the same configuration and composition as the first optical fiber.
Owing to the above dispersion control, satisfactory transmission characteristics can be realized even on the long haul transmission. Furthermore, the maintenance control becomes easier because only two kinds of the optical fibers are used.
Preferably, the distance of the wide area dispersion compensating span is substantially equal to that of the local dispersion compensating span. Accordingly, optical amplifiers of the same configuration can be used for both spans. This also makes the maintenance control easier.
Preferably, the average chromatic dispersion after the dispersion compensation by the second optical fiber at the local dispersion compensating span should be between xe2x88x924 ps/nm/km and xe2x88x921 ps/nm/km. This can realize high speed and large capacity WDM transmission on the long haul transmission of 1000 km or more.